An object of the invention is a machining method which is particularly applicable to panels of the metal type, with or without stiffeners, or composites, or of the “sandwich” type, on which machining operations involving their thickness are to be performed, such as surfacing or the formation of recesses or cells, or trimming or drilling operations.
More particularly, the panels are skin panels having a double curvature, mainly located on an aircraft's nose cone. These panels, generally made of a light alloy, range from 1 to 12 mm thick depending on the aircraft and the constituent materials of the panel (aluminum alloy, titanium alloy, and metallic or organic resin based composite).
If the material allows, the production of these panels requires shaping by drawing on a necessarily convex mold, while panels of composite material are shaped by draping-gluing-infusion and compaction methods.
Because of its productivity and its flexibility, shaping by drawing is mainly employed.
This type of shaping is performed by means of a combination of traction on the panel and “envelopment” of the convex mold so that the geometrically known shape of the panel (that which was in contact with the drawing mold) is the internal (concave) surface. The drawing process generates a plastic deformation on the entirety of the panel's thickness and consequently leads to a thinning of the section through necking. Due to the nondevelopable nature of the shape, this “loss of thickness” is not uniform over the panel's entire surface.
The known methods for mechanical machining for thickness machining operations on such panels having a double curvature such as those mentioned above consist of placing and holding them in position either on rigid tools or on beds of suction cups. In both cases, this hyperstatic positioning does not make it possible to support the panels at all points. This imperfect positioning has two consequences on the quality and the performance of mechanical machining:                It is impossible to support high cutting stresses, which reduces productivity.        No physical reference to the points which are not in contact with the positioning element is available and it is consequently impossible to obtain precise dimensions, in particular the thickness, without having recourse to complex measurement systems.        
Other problems are encountered in machining, in addition to the problem of referencing the panel.
It is, in particular, possible to produce recesses while milling complex concave surfaces by sweeping the surface, mainly with cutters having spherical ends or toroidal cutters. The quality constraints define a maximum roughness as well as a maximum allowable jog between sweeping passes. In the case of parts affected by fatigue, as is the case with aeronautical pans, these criteria are strict and the order of magnitude of these requirements is 1.6 μm in Ra for roughness and 0.04 mm for the jog tolerance.
Obtaining such criteria with a cutter having a spherical end implies the use of closer spaced sweeping cuts and therefore a reduced productivity. In addition, since the center part of the cutters having pherical ends are moved at a cutting speed of zero, the removal of material occurs there under very poor conditions, which degrades the resulting surface quality.
To resolve this disadvantage, it suffices to introduce a de-chucking, typically between 5 and 20°, so that the center of the tool is no longer in contact with the machined surface. However, this practice poses problems with local management of tool accessibility and collisions, perpendicular to the surface, problems which burden the preparation phase of the parts.
Toroidal cutters make it possible to use bigger sweeping cuts with equivalent jogs, i.e. a sweeping cut equal to the diameter of the cutter minus the diameter of the torus makes it possible, in theory, to obtain a zero jog, which provides a double advantage:                higher productivity (directly proportional to the sweeping cut size), and        the center of the tool is not involved, resulting in a better surface quality and an increased lifespan of the tool.        
On the other hand, the use of a toroidal tool poses a problem in following the trajectory and of geometrical “over-cutting” which is also called “gouging” or “heeling.” This over-cutting varies depending on the radius of the trajectory, and its variability thus ends up being added to the dispersions of various origins relating to the thickness tolerance at the bottom of a hole. To solve this problem, it is also possible to de-chuck the tool, but the trajectory corrections are complex in the case of a toroidal tool.